1. Field of the Invention
This invention relates to an etching apparatus for etching semiconductor wafers (hereinafter referred to as wafers) through wet chemical processing by exposing the wafers to a mixed acid with dispersed air bubbles for bubbling therein and, more particularly, to an etching apparatus of the above type, which is provided with a mixed acid supply method for permitting stable flatness and luster (brightness) of wafers to be obtained.
2. Description of the Prior Art
Semiconductor silicon wafers are manufactured by slicing a silicon single crystal ingot into the form of wafers, chamfering and rounding the edge of the sliced wafers, lapping the chamfered wafers to approximate a prescribed wafer thickness, etching away a roughness of the surface resulting from the lapping, and then mirror finishing a single-sided principal surface of the resultant wafers.
The etching step in the above manufacture has an aim of removing a roughness of the surface and/or surface strain layer, which are formed on the wafer surface by chamfering, lapping or like mechanical process, through dissolution in the mixed acid. For this step, an apparatus for chemically processing wafers has been desired, which can satisfy both flatness and luster.
As shown in the schematic sectional view of FIG. 5, a prior art etching apparatus comprises a mixed acid feeder 50, an etching trough 51, an overflow trough 52, and a bubbling pipe 14. Between the etching trough 51 and the mixed acid feeder 50, a rectifying plate 53 is provided. As shown in FIG. 6, the rectifying plate 53 has vertical breakthrough hole group 55 formed substantially its entire surface. Mixed acid 18 is supplied from the mixed acid feeder 50 through the rectifying plate 53 to the etching trough 51 to form a parallel laminar flow (i.e., a parallel laminar flow is constituted by a number of laminar flows in the form of slits which are parallel in the horizontal direction) 56. Air bubbles for bubbling 15 are dispersed uniformly in the parallel laminar flow through the bubbling pipe 14. In the chemical processing zone thus formed, wafers 21 are dipped such that they are parallel to the parallel laminar flow and spaced apart vertically at an adequate interval. For the chemical processing, the wafers 21 are rotated in the direction of arrow A. The mixed acid that has formed the parallel laminar flow, is allowed to overflow over a vertical wall 57, which faces the rectifying plate 53, into the overflow trough 52.
As an example of etching using the etching apparatus having the above structure, silicon wafers having a diameter of 150 mm were used by using a mixed acid containing 50% fluoric acid and equal volume ratios of concentrated nitric acid and concentrated acetic acid and setting the etching temperature to 30.degree. C.
Silicon was dissolved in advance in a range of 10.0 to 13.0 g/l in the mixed acid. The mixed acid containing silicon dissolved in this way, was supplied at a rate of 250 l/minute by a pump P through the rectifying plate 53 in the mixed acid feeder 50 to the etching trough 51, while supplying air bubbles for bubbling at a rate of 200 l/minute from a compressor C through the bubbling pipe 14 which was disposed in a bottom portion of the etching trough 51. FIG. 7 shows the results of etching the silicon wafers in the above way.
By increasing the content of dissolved silicon in the mixed acid as an etching condition in steps each of 0.5 g from 10.0 g/l to 13.0 g/l, the etching rate was reduced from 1.13 .mu.m/minute (with mixed acid containing 10.0 g/l of dissolved silicon) to 0.82 .mu.m/minute (with mixed acid containing 13.0 g/l of dissolved silicon). The luster was reduced by 47% from 62% to 33%. This luster reduction is extreme, indicating that the luster is unstable.
The fact that the luster is extremely reduced and unstable is thought to be due to the following problems present in the chemical processing zone in the prior art etching apparatus, in which the wafers are dipped.
a) In the chemical processing zone, in which the mixed acid flows in a status as shown in FIG. 5, a portion of the mixed acid corresponding to an upper portion of the chemical processing zone is caused to flow out quickly from the etching trough 51 over to the overflow trough 53, and this flow has an effect of pressing and making stagnant the mixed acid in a lower part of the zone.
b) From this reason, air bubbles for bubbling 15 are not uniformly dispersed over the entire zone.
c) The flow of mixed acid in the locality where the wafers 21 are dipped is unstable, that is, the parallel laminar flow 56 strikes perpendicularly the vertical wall 57 facing the rectifying plate 53, thus disturbing the flow of the mixed acid.
d) A great stagnated region of mixed acid is present in correspondence to a lower portion of the vertical wall 57.